Soldering

ABSTRACT

The invention features soldering parts (e.g., printed circuit boards) by indexing the parts sequentially and continuously through a series of processing stations. Flux may be applied to precise areas of the parts at a flux station, and the flux station may include a rough part locator that roughly locates a part indexed into the flux station and a precise part locator that precisely locates the part. The flux station may further include a guide rail for supporting the part and a mechanism for raising and lowering the guide rail. The flux station may include a flux sprayer for applying flux to the part, where the flux sprayer includes an air valve, a flux valve, and a controller coupled to activate and deactivate separately the air valve and the flux valve. The controller may activate the air valve before activating the flux valve and deactivate the flux valve before deactivating the air valve.

This application is a divisional of application Ser. No. 08/931,913,filed Feb. 24, 1997, now issued as U.S. Pat. No. 5,941,444, which is adivisional of application Ser. No. 08/640,018, filed Apr. 30, 1996, nowissued as U.S. Pat. No. 5,642,850 on Jul. 1, 1997, which is a divisionalof application Ser. No. 08/420,553, filed Apr. 11, 1995, now issued asU.S. Pat. No. 5,560,537 on Oct. 1, 1996.

BACKGROUND

This invention relates to soldering.

In one method of soldering, printed circuit boards (PCBs) which arepopulated with components pass, one at a time, through a three stepprocess: flux is applied to electrical connection points on both the PCBand the components; the PCB and the components are preheated; and theelectrical connection points are brought in contact with molten solder.

SUMMARY

In general, in one aspect, the invention features soldering parts (e.g.,printed circuit boards) by indexing the parts sequentially andcontinuously through a series of processing stations. Each of the seriesof processing stations includes at least one processing position, andthe indexer indexes the parts from processing position to processingposition. At different processing stations, the parts indexed into thoseprocessing stations are simultaneously processed.

Implementations of the invention may include one or more of thefollowing features. The indexer may have a chain and flights connectedto the chain and separated along the chain by an indexing distance.Solder may be applied to the parts at a soldering station. The solderingstation may include a solder fountain. The solder fountain may have asolder manifold with a first aperture, a solder well plate with secondapertures, where the solder well plate is mounted above the soldermanifold with the second apertures above the first aperture, modularsolder well plates mounted above the second apertures, and a solderchimney mounted to each of the modular solder well plates. The solderchimneys may provide passageways from the solder manifold to a top ofthe solder chimneys where the parts are soldered, the parts may be ofdifferent part types, and each of the modular solder well plates maycorrespond to one of the different part types. The solder fountain mayalso include another solder chimney mounted to each of the modularsolder well plates. The soldering station may further include a roughpart locator that roughly locates a part indexed into the solderingstation and a precise part locator that precisely locates the part. Thesoldering station may also include a guide rail for supporting the part,and a mechanism for raising and lowering the guide rail. Additionally,flux may be applied to precise areas of the parts at a flux station, andthe flux station may include a rough part locator that roughly locates apart indexed into the flux station and a precise part locator thatprecisely locates the part. The flux station may further include a guiderail for supporting the part and a mechanism for raising and loweringthe guide rail. The flux station may include a flux sprayer for applyingflux to the part, where the flux sprayer includes an air valve, a fluxvalve, and a controller coupled to activate and deactivate separatelythe air valve and the flux valve. The controller may activate the airvalve before activating the flux valve and deactivate the flux valvebefore deactivating the air valve. Furthermore, the parts may bepreheated and glue used to mount components to the parts may be cured inan oven. The oven may include sparging tubes for blowing heated gas onthe parts as the parts are indexed to an exit end of the oven. The ovenmay also have a guide rail for supporting the parts as they are indexedthrough the oven and a support rail for supporting the guide rail, wherethe support rail includes thermal expansion slots for allowing thesupport and guide rails to thermally expand longitudinally and where thesupport rail is held in a fixed position at one end and is free toexpand away from the fixed position. Moreover, the processing stationsmay include an identification station having a sensor for determining atype of a part indexed into the station, and the soldering system mayinclude a controller for controlling subsequent processing stations inresponse to the sensor. The controller may control the amount of timeflux is sprayed at each of the parts according to the identified parttype, and the controller may control the pump speed of a solder pumpaccording to the identified part type. The parts may be printed circuitboards, and the parts may include pallets having an aperture for holdingone of the printed circuit boards. The pallets may include additionalapertures for holding additional printed circuit boards.

In another aspect, the invention features a solder fountain. The solderfountain includes a solder manifold for containing solder, and thesolder manifold has a first aperture. The solder fountain also includesa solder well plate having second apertures, where the solder well plateis mounted above the solder manifold with the second apertures above thefirst aperture. Further, the solder fountain includes modular solderwell plates mounted above the second apertures, and a solder chimneymounted to each of the modular solder well plates. The solder chimneysprovide passageways from the solder manifold to a top of the solderchimneys where solder is applied to the parts. The parts are ofdifferent part types, and each of the modular solder well platescorresponds to one of the different part types.

Implementations of the invention may include one or more of thefollowing features. The chimney passageway may be an unrestrictedpassageway. The solder fountain may include a rough part locator havinga stopper arm with an end that provides a datum point and a pusher armfor pushing the part against the stopper arm and a precise part locatorhaving a datum bushing and a slotted bushing on the part and a datum pinand an expansion pin for respectively engaging the datum bushing and theslotted bushing.

In another aspect, the invention features a flux unit for applying fluxto precise areas of printed circuit boards. The flux unit includes arough part locator that roughly locates a printed circuit board withinthe flux unit and a precise part locator that precisely locates theprinted circuit board.

Implementations of the invention may include one or more of thefollowing features. The rough part locator may include a stopper armhaving an end that provides a datum point and a pusher arm for pushingthe part against the stopper arm. The precise part locator may include adatum bushing and a slotted bushing on the printed circuit board and adatum pin and an expansion pin for respectively engaging the datumbushing and the slotted bushing.

In another aspect, the invention features a flux unit for applying fluxto precise areas of printed circuit boards. The flux unit includes aflux sprayer for applying flux to the printed circuit boards, where theflux sprayer includes an air valve, a flux valve, and a controllercoupled to activate and deactivate separately the air valve and the fluxvalve.

In another aspect, the invention features a convection oven forpreheating printed circuit boards. The oven includes multiple sequentialindexed positions, a nitrogen input mechanism for providing a nitrogenenvironment, and a pair of parallel guide rails for supporting theprinted circuit boards within the oven. The oven also includes spargingtubes for blowing heated nitrogen on a printed circuit board at an ovenexit and a pair of parallel support rails for supporting the pair ofguide rails. The support rails include thermal expansion slots forallowing the support and guide rails to thermally expand longitudinallyand a fastening mechanism extending through the slots for maintaining aset width between the guide rails.

In another aspect, the invention features a method for use in connectionwith soldering printed circuit boards of different types moving along anautomated production line. The method includes identifying a type ofeach of the printed circuit boards that approaches a flux station on theproduction line and applying flux to a specific area of the printedcircuit board for an amount of time corresponding to the type of theboard.

In another aspect, the invention features a method for use in connectionwith soldering printed circuit boards of different types moving along anautomated production line. The method includes identifying a type ofeach of the printed circuit boards that approaches a solder station onthe production line and setting a solder pump speed appropriate toproviding a stable top surface of a column of solder of a predeterminedheight in a solder fountain used to apply solder to a specific area ofthe printed circuit board.

Advantages of the invention may include one or more of the following.The amount of handling required to solder components to printed circuitboards (PCBs) is reduced and the number of PCBs soldered in a givenperiod of time is increased. The PCBs are uniformly heated and notsubject to damaging temperatures. The top surface of solder columns arestable which permits precise soldering of multiple PCB areas whilepreventing solder from spreading to other closely spaced PCB areas.

Other advantages and features will become apparent from the followingdescription and from the claims.

DESCRIPTION

FIGS. 1a and 1 b are side and top views, respectively, of a continuoussoldering system.

FIG. 2a is a cut-away view at 2 a—2 a in FIG. 1a.

FIGS. 2b and 2 c are cross-sectional views at 2 b—2 b and 2 c—2 c,respectively, in FIG. 2a.

FIG. 2d is an enlarged view of a chain tensioner in FIG. 2a.

FIG. 3 is a perspective view of a pallet loaded with printed circuitboards (PCBs).

FIGS. 4a and 4 b are cross-sectional views of an identification stationat 4 a—4 a and 4 b—4 b, respectively, in FIG. 1a.

FIG. 5 is a block diagram of a control system for a continuous solderingsystem.

FIGS. 6a and 6 b are cross-sectional views of a flux unit at 6 a—6 a inFIG. 1b and 6 b—6 b in FIG. 1a.

FIG. 6c is a top view of a pallet not loaded with PCBs over a flux mask.

FIG. 7a is a block diagram of a flux sprayer.

FIG. 7b is a block diagram of a nozzle.

FIG. 8a is a cross-sectional view of an oven at 8 a—8 a in FIG. 1a.

FIGS. 8b and 8 d are cross-sectional views at 8 b—8 b and 8 d—8 d,respectively, in FIG. 8a.

FIG. 8c is a perspective view at 8 c—8 c in FIG. 8a.

FIG. 9 is a perspective view of a pallet and a slot and a shoulder screwin a support rail.

FIGS. 10a and 10 b are cross-sectional views of a solder station at 10a—10 a in FIG. 1a and 10 b—10 b in FIG. 1b, respectively.

FIG. 10c is a top view of a pallet not loaded with PCBs over a solderfountain.

FIG. 10d is a perspective view of a solder fountain.

FIG. 11a is a perspective view of a solder fountain with top surfaces ofmolten solder columns extending above solder chimneys.

FIG. 11b is a perspective view of a pallet loaded with PCBs over thesolder fountain of FIG. 11a.

FIG. 11c is a cross-sectional view of a PCB in contact with top surfacesof solder columns extended above two solder chimneys.

Referring to FIGS. 1a and 1 b, a system 10 for continuously solderingprinted circuit boards (PCBs) includes a flux station 12, an oven 14,and a solder station 16. System 10 also includes a controller 18, forexample, a programmable logic controller, of the type manufactured byAllen-Bradley Corp., Highland Heights, Ohio, USA, that monitors andcontrols the operation of system 10 and a solder feed unit 20 coupled tosolder station 16 which, under control of controller 18, feeds solder tosolder station 16.

Referring to FIGS. 2a-2 c, flux station 12, oven 14, and solder station16 are interconnected by two parallel chains 22. Chains 22 are supportedon chain guides 24 which are connected to mounting rails 26, and atpredetermined distances, e.g., 8 inches, along each chain, a flight 25is mechanically connected to the chain. The chains 22 are indexedforward (arrow 21) periodically, e.g., every 10 seconds, by thepredetermined distance, e.g., 8 inches, between the chain flights tolocate each flight and its neighbor on either end of an indexed chainposition, e.g., 14 a, within system 10. When system 10 is powered up,controller 18 advances chains 22 until home sensor 23 (e.g., a proximitysensor of the kind manufactured by Turck, Inc.) detects a leading edgeof a flight. Chains 22, chain guides 24, mounting rails 26, and flights25 are stainless steel.

As chains 22 pass through oven 14 they are heated and, although they arestainless steel, which has a low thermal expansion coefficient, theyexpand with the heat. Chains 22 also stretch with age. A weighted chaintensioner 27 (FIGS. 2a and 2 d) is mechanically coupled to chains 22,through sprockets 29 a, 29 b, 29 c, and 29 d, and together with a weight31, the chain tensioner pushes down on the chains to maintain a constantchain tension (arrows 33 a and 33 b) regardless of temperature or age.

Referring to FIG. 3, PCBs are carried through the system on pallets. Apallet 28 includes three apertures 30, 32, and 34. Each PCB 36, 38, and40 (shown unpopulated, i.e., without mounted components, for clarity,but which normally have mounted components) has a locating pin hole 39 aand a locating slot 39 b, and for each aperture 30, 32, and 34, pallet28 includes a tooling pin 39 c and an extension pin 39 d. Each PCB 36,38, and 40 is precisely located within apertures 30, 32, and 34,respectively, when tooling pin 39 c engages locating pin hole 39 a andextension pin 39 d engages locating slot 39 b. Further, each PCB 36, 38,and 40 is supported within apertures 30, 32, and 34, respectively, bylocating clips 42. Each clip applies force to a side of a PCB in adirection from that side of the aperture toward an opposite side of theaperture.

The PCBs may be of a single type or, as shown, may be of three differenttypes with each aperture 30, 32, 34 corresponding to a particular typeof PCB. When the PCBs are three different types, each pallet 23 isgenerally loaded with only one of the three types, which allows thesystem to be configured to tailor PCB processing in accordance with thetype of PCB loaded in each pallet.

Pallets 28 are loaded onto stainless steel guide rails 44 (FIGS. 2a, 2c) within system 10 either manually or automatically, e.g., by a Boschconveyor belt 46 which is indexed synchronously with chains 22. Pallets28 are centered on conveyor belt 46. Apertures 30, 32, and 34 (FIG. 3)are at least a distance, d, e.g., about 0.70 inches, from edges 35 and37 of pallets 28 to allow edges 35 and 37 to rest on guide rails 44. Toallow the pallets to smoothly pass from conveyor belt 46 to guide rails44, the top surface of conveyor belt 46 is approximately 0.005-0.010inches above the top surface of guide rails 44 and guide rails 44 have alead-in chamfer, e.g., of about 0.030 inches. Flights 25 engage a backedge 48 of each pallet 28 and push the pallet through system 10 aschains 22 are indexed. System 10 includes a sequence of processingstations and, at each indexed chain position, each processing stationmay execute a certain procedure, such as applying flux, solder, or heatto PCBs loaded in pallets. The index period is limited, therefore, bythe slowest procedures, for example, by the application of solder atsolder station 16, and by the time required to index chain 22, forexample, 1.5 seconds. As an example, where the distance between theflights is about 8 inches, the width W1 of the pallets 28 is about 6.30inches which provides a clearance of about 1.70 inches between pallets28. After being indexed through solder station 16, pallets 28 are pushedby flights 25 from guide rails 44 to a second indexed conveyor belt 47.The top surface of conveyor belt 47 is approximately 0.005-0.010 inchesbelow the top surface of guide rails 44 to allow the pallets to smoothlypass from guide rails 44 to belt 47.

Referring to FIGS. 4a and 4 b, within flux station 12, a first indexedchain position locates a pallet 28 at an identification (ID) station 50.ID station 50 includes four retroreflective sensors 52, 54, 56, and 58(e.g., a fiber photoelectric sensor having a transmitter, for example, alight emitting diode (LED), and a receiver). Sensor 52 detects thepresence of the pallet by receiving light reflected from the pallet.Sensors 54, 56, and 58 detect the presence of the particular PCBs 36,38, and 40, respectively, by receiving light reflected from those PCBs.

Referring to FIG. 5, the sensors within ID station 50 send electricalsignals to controller 18 to notify controller 18 that a pallet 28 ispresent and to notify controller 18 which pallet aperture(s) 30, 32, or40 contains a PCB (i.e., which PCB type(s) is present within thatpallet). If sensor 52 detects a pallet, but none of the sensors 54, 56,and 58 detect a PCB, then controller 18 indicates an error condition oncontrol screen 60 to notify an operator. The controller mayautomatically prevent chain 22 from indexing until the pallet is removedor the operator, using control screen 60 and controller 18, may manuallyprevent chain 22 from indexing until the pallet is removed.Alternatively, the controller can automatically or the operator cnmanually allow the pallet to proceed through system 10 and prevent theflux unit 62 (FIG. 6a) and solder station 16 from operating when thatpallet is indexed into those stations.

Referring to FIGS. 6a and 6 b, a second indexed chain position locatesthe pallet 28 on flux rails 61 of a flux unit 62 within flux station 12.Because chains 22 can stretch as they age and as they are heated, twopneumatic actuator arms, a stopper arm 64 and a pusher arm 66, are usedto roughly locate the pallet. When the pallet is indexed onto flux rails61, stopper arm 64 is activated and rotates (arrow 65) a predeterminedamount to position an end 68 at a datum point beyond a front edge 70 ofthe pallet, and when activated, pusher arm 66 also rotates (arrow 67) apredetermined amount to position an end 72 against and push (arrow 73)back edge 48 of the pallet such that front edge 70 contacts stopper arm64. This action positions the front edge 70 of the pallet at the datumpoint (i.e., against end 68 of stopper arm 64). Pusher arm 66 includes aspring loaded lever 74 that compresses (arrow 75) slightly when frontedge 70 of the pallet contacts stopper arm 64. spring loaded lever 74compensates for the pallet width tolerance and the rotation tolerancesof stopper arm 64 and pusher arm 66 and prevents the arms from applyingsignificant pressure to the front and back edges of the pallet.

Once the pallet is roughly located, controller 18 determines whether thepallet is properly seated on rails 61 with seating sensors 80 and 82,e.g., retroreflective sensors, located at two diagonal corners of thepallet. The amount of reflected light which should be received bysensors 80, 82 when the pallet is properly seated on rails 61 ispredetermined during a set up procedure, and if the amount of reflectionindicates, for example, that one or both corners f the pallet are apredetermined distance, e.g., 0.015 inches, or more above or below rails61, then controller 18 indicates an error on control screen 60 (FIG. 5)to notify the operator of the error. Controller 18 may thenautomatically prevent chains 22 from indexing until the sensors indicatethat the error has been fixed or the operator, through control screen 60and controller 18, may manually prevent the chain from indexing until heor she fixes the error.

Rails 61 are mechanically coupled to an axis or a rail table 63 having acentral aperture 63 a. Rail table 63 is mechanically coupled to a leadscrew (not shown) which is driven by a stepper motor 84. If the palletis properly seated on rails 61, controller 18 sends electrical signalsto stepper motor 84 to cause stepper motor 84 to turn the lead screw tolower (arrow 86) rail table 63 and rails 61 to a predetermined positionabove, approximately 1.0-1.5 inches, flux mask 88. With a lead screwhaving a repeatability of about +/−0.00039 inches and a positionaccuracy of about +/−0.0003 inches and a motor 84 such as an SX57-102motor manufactured by Compumotor, Inc. a rail height position accuracyof about +/−0.001 inches is possible.

The pallet includes a datum bushing 90 and a slotted bushing 92 (FIGS.3, 6 c) both having radiused lead-in edges. As the pallet is lowered, adatum pin 94 and an expansion pin 96, which are press fitted to fluxmask 88 and extend above flux mask 88, are inserted within datum bushing90 and slotted bushing 92, respectively. The shanks (shank 94 a of datumpin 94 is shown in FIG. 6a) are straight and round and have a taperedtop (94 b). The tapered shanks engage the radiused bushings to preciselyalign the pallet. As an example, the shank diameter of both pins 94, 96is approximately −0.187 inches and the diameter of the top of both pinsis about 0.157 inches. The datum bushing has a diameter of about 0.191inches with a {fraction (1/32)} inch radiused lead-in, while the slottedbushing has a length of about 0.234 inches, a width of about 0.191inches, and a {fraction (1/64)} inch radiused lead-in.

The combination of the datum bushing 90 and slotted bushing 92 ensuresthat the pins 94, 96 will properly mate with the bushings despitethermal expansion or contraction of the pallet. As the pallet passesthrough system 10, temperature variations may cause the pallet to expandand contract. Thermal expansion is greatest toward the areas of leastresistance, typically, edges 35, 37, 48, and 70. Because datum bushing90 is substantially centered with respect to the length L and width W1of the pallet, the potential thermal expansion of the pallet from thedatum bushing toward back edge 48 and front edge 70 of the pallet issubstantially equal and the potential thermal expansion of the palletfrom the datum bushing toward edges 35 and 37 is substantially equal. Asa result, the position of datum bushing 90 remains substantiallycentered with respect to the edges of the pallet as the pallet thermallyexpands and contracts and, therefore, very little clearance,approximately 0.004 inches, between datum pin shank 94 and datum bushing90 is required. Similarly, because slotted bushing 92 is substantiallycentered with respect to the width W1 of the pallet, the potentialthermal expansion of the pallet from the slotted bushing toward backedge 48 and front edge 70 is substantially equal and very littleclearance, approximately 0.004 inches, is required between the sides 92a, 92 b of slotted bushing 92 and the shank of expansion pin 96. On theother hand, slotted bushing 92 is not centered with respect to thelength L of the pallet and the potential for thermal expansion of thepallet from the slotted bushing toward edge 35 is far greater than thethermal expansion of the pallet from the slotted bushing toward edge 37.As a result, the length of slotted bushing 92 provides a largelength-wise clearance, approximately 0.047 inches, between expansion pin96 and the sides 92 c, 92 d of slotted bushing 92 to compensate forunequal potential thermal expansion and contraction of the palletbetween slotted bushing 92 and edges 35 and 37.

Similarly, the combination of locating pin hole 39 a (FIG. 3) andlocating slot 39 b is used to compensate for potentially unequal thermalexpansion and contraction of the pallet and a PCB located within anaperture of the pallet by tooling pin 39 c and extension pin 39 d.

Once the pallet is lowered to the predetermined distance above flux mask88 and the pallet is precisely located by pins 94, 96, controller 18checks, using seating sensors 80, 82, whether the pallet is properlyseated on rails 61. If the pallet is not properly seated, controller 18notifies the operator of the error through control screen 50 (FIG. 5)and either controller 18 automatically prevents chains 22 from indexinguntil the sensors indicate the pallet is properly seated, or theoperator, through control screen 60 and controller 18, prevents thechains from indexing until the pallet is properly seated, or the processis continued and the PCB(s) in that particular pallet is considered areject. The controller may automatically or the operator, throughcontrol screen 60 and controller 18, may manually raise and lower rails61 again to try to properly seat the pallet.

Once the pallet is properly seated, controller 18 sends electricalsignals to a flux sprayer 104 to cause flux to be sprayed upward (arrows106, FIG. 6a) toward flux mask 88 but only in the area(s) of a loadedPCB(s). Flux mask 88 can be machined from many materials, includingplastic or plated steel.

Referring to FIG. 6c, flux mask 88 includes apertures 108, 110, 112which correspond to particular areas of PCBs 36, 38, 40, respectively(not shown, for clarity) where electrical connection points betweencomponents mounted on the PCBs and the PCBs are to be soldered in solderstation 16 (FIG. 1a). When flux sprayer 104 sprays flux, the flux passesthrough apertures 108, 110, and 112 and provides a thin coat on theparticular PCB areas. Portions 114, 116, 118 of the flux mask preventflux from being applied to other areas of the PCBs.

Flux mask portions 114, 116, 118 can be divided into two or moresections of differing heights. For example, flux mask portions 114 a,116 a, and 118 a are lower (i.e., closer to flux sprayer 104) than fluxmask portions 114 b, 116 b, and 118 b. Lower flux mask portions 114 a,116 a, and 118 a can accommodate components (e.g., single turntransformer windings, not shown) that extend from a bottom surface ofthe PCBs and allow connection points located on the PCBs next to theupper flux mask portions 114 b, 116 b, and 118 b to be brought as closeas possible to flux sprayer 104.

Referring to FIG. 7a, flux sprayer 104 includes a set of two valves, apneumatic air valve 120 and a flux valve 122, for each aperture 30, 32,and 34 (FIG. 3) of pallet 28. Both valves are controlled by controller18. Flux sprayer 104 also includes a set of one or more nozzles 124directed at each aperture 30, 32, and 34 to provide an even applicationof flux to PCBs loaded with the apertures. For example, three nozzlesare directed at large PCB 36, two nozzles are directed at medium PCB 38,and one nozzle is directed at small PCB 40. Referring to FIG. 7b, eachnozzle 124 includes one flux jet 126 and multiple (four are shown, butthere may be more) air jets 128 directed at flux mask 88. Air expelledby the air Jets atomizes flux expelled by the flux jet. The controllerselectively activates only those sets of air and flux valves associatedwith apertures within which PCBs are loaded.

In typical flux stations, the air valve may be used to activate the fluxvalve: turning on the air valve turns on the flux valve; and turning offthe air valve turns off the flux valve. However, because the flux jetsmay expel flux for a short period of time after the air jets havestopped expelling air, the air jets can become clogged with flux.

In system 10, controller 18 sends electrical signals to each set of airand flux valves 120, 122, respectively, to separately control when eachvalve is turned on and off and to control which nozzles 124 receive airand flux. When rails 61 are lowered and hold a properly seated pallet,controller 18 turns on air valve 120 and after a first predeterminedamount of time, approximately 1.0 seconds, turns on flux valve 122. Fluxis sprayed for a second predetermined amount of time through selectednozzles to apply the thin layer of flux to a PCB loaded in the pallet.Flux station 62 includes an exhaust pipe 130 which allows venting offlux gasses. The second predetermined amount of time may vary dependingupon the type of PCB loaded in the pallet, for example, the secondpredetermined amount of time is about 1.5-2.0 seconds for PCB 36 (FIG.3), about 1.0-1.5 seconds for PCB 38, and about 0.5-1.0 seconds for PCB40. After the second predetermined amount of time passes, controller 18turns off flux valve 122 and waits a third predetermined amount of timefor the flux jets to finish expelling flux and for flux in thesurrounding air to settle, before turning off air valve 122. As aresult, air is continuously expelled from the air jets both before andafter flux is expelled from the flux jets to substantially prevent theair jets from becoming clogged with flux. The third predetermined amountof time may also vary with the type of PCB loaded in the pallet, forexample, where three nozzles are directed at large PCB 36 and only onenozzle is directed at small PCB 40, it may take less time, for example,about 0.6 seconds, for the three nozzles directed at large PCB 36 tofinish expelling flux than the time, for example, about 1.4 seconds, forthe one nozzle directed at small PCB 40 to finish expelling flux.Periodically and without a pallet on rails 61, controller 18 causes theair jets to expel short quick bursts of air to purge or clean the airjets which provides additional protection against clogged air jets.

After applying flux, controller 18 sends electrical signals to steppermotor 84 to cause motor 84 to turn the lead screws and raise rails 61and pallet 28 such that rails 61 are level with guide rails 44. A railhome sensor (not shown) and a rail bottom sensor (not shown) can be usedby the controller to determine if rails 61 are in a home or top positionor in a bottom or flux position, respectively. The steps of roughlylocating the pallet, lowering the rails, spraying flux, and raising therails are accomplished within the index period.

The next indexed position following the flux station indexed positionlocates the pallet in convection oven 14. Oven 14 has multiple indexedpositions, e.g., thirteen (for clarity, only four 14 a, 14 b, 14 c, and14 d are shown in FIG. 2a), and runs at a temperature of about 140° C.to slowly preheat a PCB(s) loaded in the pallet to about 105-110° C.,before the pallet is indexed into solder station 16. Preheating the PCBenhances the solder wettability of the connection points and alsoactivates the flux just applied to the connection points. Slowlypreheating the pallet and PCB to a desired temperature provides auniform temperature across the PCg and reduces the risk of damage to thePCB if controller 18 prevents chain 22 from indexing for a period oftime when an error is detected in system 10. Past systems have used hightemperature, e.g., greater than 280° C., infra-red (IR) or convectionpanels to quickly preheat individual PCBs to about 105-110° C. However,a PCB left between such high temperature panels beyond a short thresholdtime may be damaged.

Pallets 28 (FIG. 3) can be made from many materials, including hardcoated teflon and aluminum. Aluminum is a preferred pallet materialbecause aluminum pallets typically cost less to manufacture and are moreeasily detected by metal detecting proximity detectors located in otherstages of the overall PCB manufacturing system (not shown).Additionally, aluminum is heavier than teflon and may assist in seatingthe pallets on the datum and expansion pins.

Aside from preheating PCBs loaded in pallets, oven 14 can also cure theglue used to mount some components to the PCBs. This may eliminate aprior glue curing stage (not shown) in the overall PCB manufacturingsystem (not shown). If the glue curing temperature is higher, e.g., 125°C., than the PCB soldering preheat temperature, e.g., 105-110° C., thenoven 14 may need to be run at a higher temperature, e.g., 160° C., inorder to cure the glue. Care should be taken that in raising thetemperature the PCBs are not damaged and the application of solder tothe PCBs in solder station 16 is not adversely affected.

Nitrogen is continuously added, through a nitrogen input mechanism 83(FIG. 5) activated by controller 18, to oven 14 at a rate of, e.g., 20cubic feet per minute (cfm), which pushes contaminants out of the ovenand provides a nitrogen environment. An oxygen analyzer 85 (FIG. 5) isused by controller 18 to monitor the oven environment, and controller 18flags an error to the operator if the parts per million (ppm) of oxygenwithin oven 14 exceeds a predetermined threshold, for example, 100 ppm.The nitrogen environment reduces the possibility of oxides forming onPCB connection points as pallets are indexed through the oven.Additionally, because solder station 16 also has a nitrogen environment,directly connecting the oven to the solder station will not degrade thenitrogen environment of solder station 16.

At the entrance 132 (FIG. 1a) of oven 14, sparging tubes (tubes withmultiple holes; not shown in the Figure) above and below enteringpallets are used to blow nitrogen at the pallets. A nitrogen containmentcurtain (not shown) can also be draped across the entrance to preventcontaminants from entering oven 14.

Referring to FIGS. 8a and 8 b, several, e.g., ten, heated aluminumplates are distributed within and heat oven 14. Half, e.g., five, of thealuminum plates are supported within oven 14 below (for clarity, onlythree aluminum plates 138 are shown in dashed outline in FIG. 8a) guiderails 44 (FIG. 2c), which support pallets being indexed through theoven, and half of the aluminum plates (not shown) are supported withinoven 14 above guide rails 44. As the pallets are indexed through thethirteen indexed chain positions within oven 14, the pallets passbetween the top and bottom heated aluminum plates 138.

On a side of each aluminum plate 138 that is opposite to the side facingthe pallets, a nitrogen input tube 133 expels (arrow 135 a) nitrogeninto oven 14 on one side of a fan 134, and fan 134 pushes (arrows 135 b)the expelled nitrogen and environmental (i.e., already within the oven)nitrogen (arrows 135 c) against aluminum plate 138. Aluminum plate 138has many small vertical holes 136 through which the nitrogen passes andbecomes heated. Holes 136 in aluminum plate 138 provide a steady, evenflow of warm nitrogen between guide rails 44 and, hence, against palletson those rails.

Additionally, underneath the two aluminum plates 138 closest to an ovenexit chamber 148, fan 134 pushes nitrogen past tubes 144, 146, filledwith nitrogen, to heat the nitrogen in the tubes. Referring also toFIGS. 8c and 8 d, tubes 144, 146 are then passed to oven exit chamber148 where tube 144 is separated into two sparging tubes 144 a, 144 b(tubes with multiple holes) which are extended across a top of exitchamber 148 and where tube 146 becomes a sparging tube and is extendedacross a bottom of exit chamber 148. Sparging tubes 144 a, 144 b, and146 direct (lines 151) the heated nitrogen across a top and a bottom,respectively, of exiting pallets. A nitrogen containment curtain (notshown) may also be draped across exit 150 of exit chamber 148.

In system 10, exit chamber 148 is only one indexed position wide andheated nitrogen is blown across PCBs loaded in pallets to substantiallyprevent the PCBs from cooling.

Support rails 45 (FIG. 2c) in oven 14 are fixed to flux station 12within ID station 50 (FIG. 1a) to prevent longitudinal thermal expansionin a direction toward flux station 12, while guide rails 44 in oven 14are not fixed at oven exit 150 to allow for longitudinal thermalexpansion in a direction toward solder station 16. Referring to FIGS. 2aand 9, guide rails 44 in oven 14 are mounted on support rails 45 whichinclude slots 152 through which shoulder screws 154 are anchored to theoven frame. Slots 152 allow support rails 45 and guide rails 44 tothermally expand and contract longitudinally, for example, by as much asabout 0.312 inches. Shoulder screws 154 keep the rails on each side ofthe oven parallel and separated by a set width W2 (FIG. 2c) whichinsures that guide rails 44 always support pallet edges 35 and 37 andmaintain the pallets on a straight path through oven 14.

Referring to FIGS. 10a and 10 b, from exit chamber 148, the pallet 28 isindexed onto solder rails 155 of solder station 16. A stopper arm 156and a pusher arm 158 operate to roughly locate the pallet, as discussedabove with respect to stopper arm 64 (FIG. 6a) and pusher arm 66 of fluxunit 62, and seating sensors 160, 162 are used by controller 18 todetermine the proper seating of the pallet on solder rails 155, similarto the use of seating sensors 80, 82 (FIG. 6b). Solder rails 155 aremechanically coupled to a rail table 163 having an aperture 163 a. Railtable 163 is mechanically coupled to a lead screw (not shown) which isdriven by a stepper motor 164, similar to stepper motor 84 (FIG. 6b).Controller 18 uses motor 164 to lower solder rails 155, and, hence, thepallet, toward a stainless steel solder fountain 166 and to raise solderrails 155 away from solder fountain 166. A datum pin 170 and anexpansion pin 172 are press fit into and extend above solder fountain166 and are used to precisely locate the pallet, and are similar todatum pin 94 and expansion pin 96 (FIG. 6b). A nitrogen input mechanism161 (FIG. 5), controlled by controller 18, provides nitrogen to twosparging tubes 157, 159 (for clarity, shown only in FIG. 10a) whichextend across and above the solder fountain and expel nitrogen at a rateof, e.g., about 30 cfm, toward solder fountain 166.

When rail table 163 and, hence, solder rails 155 are lowered to apredetermined distance above solder fountain 166, if seating sensors160, 162 indicate an improper seating of the pallet, controller 18automatically causes stepper motor 164 to raise solder rails 155 toprevent a PCB(s) in the pallet from being damaged. Controller 18 alsoflags an error, through control panel 60 (FIG. 5), to the operator.Controller 18 may automatically prevent chains 22 from indexing andre-lower solder rails 155 or the operator, through control panel 60 andcontroller 18, may manually prevent chains 22 from indexing and re-lowersolder rails 155. Alternatively, the PCB loaded in the pallet can beconsidered a reject.

Referring to FIGS. 10c and 10 d (for clarity, pins 170, 172 are notshown in FIG. 10d), solder fountain 166 includes a solder manifold 174(i.e., main solder chimney) filled with solder (not shown) and a solderwell plate 176. Solder well plate 176 includes modular solder wells 178,180, and 182 which correspond to PCBs 36, 38, 40 (FIG. 3). The modularsolder wells are mounted over apertures (not shown) in solder well plate176. These mounted modular solder wells can be replaced with redesignedmodular solder wells. For instance, if a PCB is redesigned and thelocations of connection points are changed, a redesigned modular solderwell corresponding to the redesigned PCB can mounted to the solder wellplate in place of the existing modular solder well.

Each modular solder well 178, 180, 182 includes multiple solder chimneys184 which correspond precisely to PCB areas having connection points tobe soldered. A heater 185 (FIG. 5) heats the solder within soldermanifold 174 such that the solder flows freely. Each solder chimney 184provides an open, unrestricted passageway (P, FIG. 11c) from soldermanifold 174 to a top of the solder chimneys 184 to allow for the flowof solder.

Pumping nitrogen into the solder fountain reduces the amount ofcontaminants, e.g., oxygen, and, therefore, prevents the flowing solderfrom oxidizing. An oxygen analyzer 187 (FIG. 5) is used by controller 18to monitor the parts per million (ppm) of oxygen in solder station 16.Controller 18 flags an error to the operator through control screen 60if the level of oxygen exceeds a predetermined threshold, e.g., 100 ppm.

Referring to FIGS. 11a-11 c (pins 170, 172 are not shown in FIG. 11a andPCBs 36, 38, and 40 are shown without mounted components in FIG. 11b),when a pallet 28 is lowered on solder rails 155 (not shown), controller18 sends electrical signals to pump 183 (FIG. 10b) to cause pump 183 topump solder from solder manifold 174 through chimneys 184 at a solderingpump speed suitable to form stable surfaces 190 on the tops of moltensolder columns passing through solder chimneys 184. The pump speeddepends upon the size and characteristics of the PCB being soldered, forexample, the soldering pump speed for PCB 36 is, for example,approximately 80% of pump capacity, while the soldering pump speed forPCB 40 is, for example, approximately 78% of pump capacity. The areas ofa PCB having connection points are then brought in contact, for apredetermined amount of time, e.g., about 5.5-5.7 seconds, with topsurface 190. The solder wets to the fluxed and pre-tinned (i.e.,pre-treated) connection points between the PCB and components (notshown) mounted on the PCB.

Referring to FIG. 11c, when areas of PCB 36 are brought in contact withtop surfaces 190, the solder wets to pre-treated connection points 191a-191 g. Those connection points which lie directly over a chimney aresoldered whereas those which do not lie above a chimney are not: thus,connection point 191 a is precisely soldered while an adjacent, extendedcomponent, e.g., a single turn transformer winding 196, is not. Theconnection points can lie flush with a bottom side 36 a of PCB 36, suchas 191 a, 191 c, 191 e, and 191 g, or the connection points can lie on atop surface 36 b of PCB 36 and adjacent to through-holes 194 in PCB 36.For instance, a surface mount component 195, e.g., a capacitor, has aconnection point 191 b on top surface 36 b and adjacent to through-hole194. When the bottom surface of PCB 36 is brought in contact with topsurface 190, the solder wets to through-hole 194 and to connection point191 b, as described in U.S. patent application Ser. No. 08/225,263,filed Apr. 8, 1994, and assigned to the same assignee as thisapplication and U. S. patent application Ser. No. 08/337,245, filed Nov.10, 1994, and also assigned to the same assignee as this application.Additionally, connection points 191 d and 191 f of components 197 a and197 b are leads which extend through adjacent through-holes 194, andwhen bottom surface of PCB 36 is brought in contact with top surface190, the solder wets to connection points 191 d and 191 f andthrough-holes 194. The leads can extend partially into through-hole 194,completely through through-hole 194 and a small distance, e.g., 0.010inches, beyond bottom surface 36 a, e.g., connection point 191 d, orcompletely through through-hole 194 and significantly, e.g., 0.10inches, beyond bottom surface 36 a, e.g., connection point 191 f.

One type of solder fountain provides small inlets at a bottom of solderchimneys between the solder manifold and the chimneys. The flow ofsolder through these inlets may disrupt the top surface of the moltencolumn of solder passing through the chimneys, and, as a result,vibrating or unstable top surfaces may apply solder beyond the PCB areascontaining connection points when the PCB is brought in contact with thetop surfaces.

Stable top surfaces 190 allow connection points in precise locations tobe soldered. The unrestricted passageways, P, between solder manifold174 and the tops of chimneys 184 may avoid the disruption that may becaused by the flow of solder through inlets in the bottom of thechimneys. Additionally, solder fountain 166 is heavy, e.g.,approximately 160-200 lbs, which tends to dampen vibrations in system 10and substantially prevent the vibrations from passing through solderfountain 166 and disrupting the stable top surfaces 190 of the moltensolder columns passing through the chimneys. Solder fountain 166 is alsoleveled, to approximately 0.005 inches or better, with a precision gradelevel to insure that the stable top surfaces 190 are also level.

Stepper motor 164 (FIG. 10b), like stepper motor 84 (FIG. 6b), bringssolder rails 155 to a predetermined height with an accuracy ofapproximately +/−0.001 inches. Using pump speed, solder temperature, andsolder level, controller 18 can estimate the height of top surfaces 190above chimneys 184. The solder pump preferably includes a closed looprotation per minute (rpm) controlled motor system which preciselymaintains a selected motor speed. Through control panel 60 (FIG. 5), theoperator selects a pump speed and, in response, controller 18 sends anelectrical signal to a motor controller (not shown) which corresponds tothe selected pump speed. The motor controller uses the electrical signalto set the motor rpm to the desired level and uses an rpm sensor (notshown) to detect the actual speed of the motor. The motor uses thedetected actual motor speed to maintain the motor speed at the desiredlevel.

Instead of estimating the height of top surfaces 190, a method foraccurately determining the height to which solder rails 155 should belowered begins by loading a high temperature glass plate, e.g.,neoceran, into one or more of the apertures in a pallet. The pallet isthen placed on solder rails 155 and rails 155 are lowered until theoperator visually detects that the glass plate has come in contact withtop surfaces 190 and a consistent displacement of solder is detectedacross the glass plate at the location of each chimney. A teach buttonon control panel 60 (FIG. 5) can then be activated by the operator toset the height to which controller 18 will cause stepper motor 164 tobring rails 155.

The height to which rails 155 are brought and the pump speed of the pumpmay vary with the type of PCB loaded in the pallet. For example, smallPCBs, e.g., PCB 40, often use modular wells, e.g, modular well 182, withthicker walled, smaller chimneys 184. The small amount of solder passingthrough thick walled, small chimneys may heat the chimneys less than thelarger amount of solder passing through thinner walled, larger chimneys,e.g., modular well 178. Consequently, the pump speed may have to beincreased to pass the solder through these smaller chimneys and achievethe same top surface 190 height. The operator can manually or controller18 can automatically set three different pump speeds depending uponwhich PCB is loaded in the pallet.

After a PCB in a pallet has been soldered and solder rails 155 raise thepallet, or when solder rails 155 do not hold a pallet, controller 18(FIG. 5) sends electrical signals to a solder pump 183 (FIGS. 5 and 10b)to cause solder pump 183 to pump solder from solder manifold 174 throughsolder chimneys 184 at an overflow rate, e.g., 85% of pump capacity, orto clean out the solder chimneys, controller 18 periodically sendselectrical signals to solder pump 183 causing solder pump 183 to pumpsolder from solder manifold 174 through solder chimneys 184 at a purgerate, e.g., 90-95% of pump capacity. The solder which overflows thechimneys is caught by weirs 186 and recirculated to solder manifold 174.

The flowing, heated solder heats solder well plate 176 and chimneys 184and also heats the environmental nitrogen. The solder contained withinsolder manifold 174 provides a certain heat mass, and the larger thismass, the less likely it will be that heat transferred to the solderwell plate, chimneys, and environmental nitrogen will affect the overalltemperature of the solder. To direct the overflow of solder and preventundirected overflow paths from applying solder to undesired areas of thePCB, overflow indentations 188 (FIGS. 10d, 11 c, and 11 c) can bemachined into one or more of the chimneys (for clarity, only one isshown).

After a certain number of pallets have passed through system 10,controller 18 detects the level of solder in manifold 174 with, e.g., aretroreflective solder level sensor 192 (FIG. 10a) or a floating solderlevel sensor (not shown). When the level of solder is below a thresholdlevel (determined by the placement of solder level sensor 192)controller 18 sends electrical signals to solder feed unit 20 (FIG. 1b)to cause solder feed unit to add solder to solder station 16.

In a partially loaded system 10 (i.e., only three pallets are shownwithin system 10 in FIG. 2a), controller 18 causes the procedures atflux unit 62 and solder station 16 to be executed only when a palletwith a loaded PCB is located at that station. When system 10 is fullyloaded (i.e., one pallet with a loaded PCB is located between each pairof flights), controller 18 causes the procedures at flux unit 62 andsolder station 16 to be executed during each index period.

Other embodiments are within the scope of the following claims.

For example, although controller 18 was described as providing differentflux spray times and different solder pump speeds for the threedifferent types of PCBs which may be loaded in a pallet, a single fluxspray time and a single solder pump speed may be sufficient for allthree PCB types. If so, two or all three types of PCBs may be loaded atone time in a pallet as the pallet is passed through system 10.Similarly, the pallet can be designed to hold one PCB or one or more ofthe same type of PCB.

Controller 18 may include a central controller electrically connected toprocessing station sub-controllers, for instance, identification station50 (FIG. 1b), flux unit 62, oven 14, and solder station 16, may eachinclude a sub-controller specifically designed to control the operationof the station. The central controller monitors the operation of theentire system and coordinates the operation of the sub-controllers.

A motor 198 (FIG. 10b) which drives chains 22 can include a slip clutch199, e.g., a torque limiter clutch. With a slip clutch, if chains 22 areprevented from indexing, for instance, by a jammed pallet, the clutchslips and, as a backup, motor electronics limit the current to the motorto prevent the motor from forcing chains 22 to index which may damagechains 22, system 10, or a jammed pallet. Controller 18 can monitorchain movement by determining whether a flight 25 periodically passeshome sensors 23 (FIG. 2a). If flights are not detected within apredetermined amount of time, then controller 18 flags an error to theoperator.

The walls of chimneys 184 can be machined to change the shape of thechimneys and the wall thickness. This may be required for minorspecification changes or if solder is not being properly applied todesired PCB areas. For more significant specification changes, a modularwell 178, 180, or 182 (FIG. 10d) can be removed from well plate 176 andreplaced with a new modular well reflecting the specification changes.Additionally, a modular well corresponding to a different PCB typealtogether may replace one of the existing modular wells attached towell plate 176. Similarly, well plate 176 can be replaced with a newwell plate having different sized apertures, possibly in differentlocations, for receiving different sized modular wells. of course, suchchanges may required similar changes to pallet 28.

Identification station 50 could include a bar code scanner (not shown)for reading a bar code 200 (FIG. 3) on pallet 28 and for notifyingcontroller 18 of the results of the bar code scan. Controller 18 couldthen use the results of the bar code scan to determine using, forexample, a table look up, which PCB or PCBs are loaded in pallet 28. Barcodes allow controller 18 to keep track of individual pallets.

Many sensors can be placed throughout system 10 and monitored bycontroller 18 to detect error conditions. For example, a soldertemperature sensor 202 (FIG. 10a) can be monitored by controller 18 toprevent controller 18 from activating the solder pump, and possiblydamaging the solder pump, when the solder is below a first predeterminedtemperature, e.g., 230° C. Similarly, solder temperature sensor 202 canbe monitored by controller 18 to prevent controller 18 from activatingthe solder pump, and possibly damaging a PCB loaded in a pallet onlowered solder rails 155, when the solder temperature is above a secondpredetermined temperature, e.g., 270° C.

Upon the detection of an error condition, controller 18 can executeautomatic procedures or wait for instructions from the operator. Whencontroller 18 or an operator prevents chains 22 from indexing,controller 18 can automatically or the operator can manually reduce thetemperature at which oven 14 is running to further reduce thepossibility that PCBs being indexed through oven 14 will be heatdamaged. If controller 18 is connected to conveyor belt 46 (FIG. 1b),and controller 18 detects an error, controller 18 can automatically stopconveyor belt 46 and prevent new pallets from entering system 10. Asidefrom notifying the operator of errors through control panel 60 (FIG. 5),controller 18 could notify the operator through alarm lights or bells.

What is claimed is:
 1. A flux unit for applying flux to precise areas ofprinted circuit boards comprising a flux sprayer for applying flux tothe printed circuit boards, the flux sprayer including an air valve, aflux valve, and a controller coupled to activate and deactivateseparately the air valve and the flux valve.
 2. The flux unit of claim1, wherein the controller activates the air valve before activating theflux valve and deactivates the flux valve before deactivating the airvalve.
 3. The flux unit of claim 1 further comprising a rough partlocator that roughly locates the printed circuit board within the fluxunit, and a precise part locator that precisely locates the printedcircuit board.
 4. The flux unit of claim 3, wherein the rough partlocator includes a stopper arm having an end that provides a datumpoint, and a pusher arm for pushing the part against the stopper arm. 5.The flux unit of claim 3, wherein the precise part locator includes adatum bushing and a slotted bushing on the printed circuit board, and adatum pin and an expansion pin for respectively engaging the datumbushing and the slotted bushing.
 6. The flux unit of claim 3, furthercomprising a guide rail for supporting the printed circuit board, and amechanism for raising and lowering the guide rail.
 7. The flux unit ofclaim 2 further comprising a rough part locator that roughly locates theprinted circuit board within the flux unit, and a precise part locatorthat precisely locates the printed circuit board.
 8. The flux unit ofclaim 7, wherein the rough part locator includes a stopper arm having anend that provides a datum point, and a pusher arm for pushing the partagainst the stopper arm.
 9. The flux unit of claim 7, wherein theprecise part locator includes a datum bushing and a slotted bushing onthe printed circuit board, and a datum pin and an expansion pin forrespectively engaging the datum bushing and the slotted bushing.
 10. Theflux unit of claim 7, further comprising a guide rail for supporting theprinted circuit board, and a mechanism for raising and lowering theguide rail.
 11. A flux unit for applying flux to precise areas ofprinted circuit boards comprising a flux sprayer for applying flux tothe printed circuit boards, the flux sprayer including an air valve, aflux valve, and a controller coupled to activate and deactivateseparately the air valve and the flux valve, the controller activatesthe air valve before activating the flux valve and deactivates the fluxvalve before deactivating the air valve, a rough part locator thatroughly locates the printed circuit board within the flux unit, therough part locator including a stopper arm having an end that provides adatum point and a pusher arm for pushing the part against the stopperarm, and a precise part locator that precisely locates the printedcircuit board, the precise part locator including a datum bushing and aslotted bushing on the printed circuit board, and a datum pin and anexpansion pin for respectively engaging the datum bushing and theslotted bushing.
 12. The flux unit of claim 11, further comprising aguide rail for supporting the printed circuit board, and a mechanism forraising and lowering the guide rail.